Head for string trimmer

ABSTRACT

A cutting head for a string trimmer is provided which accommodates any string gauge thickness and may be easily and quickly re-threaded upon failure of a string previously disposed therein. Clamping members are provided to clamp the strings within the cutter head, with the clamping force being provided by springs and centrifugally generated moments. The clamping members are capable of generating two degrees of clamping force. The lesser degree of clamping force can be overcome by a string forcibly inserted into the cutting head.

This application is a continuation of U.S. Application Ser. No.08/709,193 filed Aug. 23, 1996, now U.S. Pat. No. 5,758,424.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to garden string trimmers, and moreparticularly, to a cutting head for string trimmers which can be easilyre-strung.

2. Description of the Prior Art

String trimmers are well known in the prior art. The typical prior artstring trimmer comprises a planar cutting head with at least tworadially extending strings which is rotated at a sufficient velocity tocause the strings to stiffen. The strings are formed usually fromplastic and are sufficiently strong enough to cut through blades ofgrass, hedges, and other vegetation in a stiffened state. The strings,however, will break upon hitting stone, concrete or some other hardmaterial commonly found in a garden setting.

String trimmers are most often used to cut vegetation along a border ofa flower bed or plot of grass which is adjacent to a sidewalk, driveway,or solid structure. Since the cutting head rotates at a speed that makesthe strings nearly imperceptible, an operator of the string trimmer isnot always able to see that the strings may come into contact with ahard surface and react accordingly. As a result, strings on a stringtrimmer are commonly broken. Prior art designs of string trimmer cuttingheads have attempted to ease the re-stringing process which may be quitetime consuming. One type of cutter head design includes an array ofgrooves and cut-outs in the hub of the cutting head which act to anchora string therein as in U.S. Pat. No. 4,190,954 to Walto entitled"CUTTING HEAD" which issued Mar. 4, 1980, U.S. Pat. No. 4,905,465 toJones, entitled "ROTARY CUTTING MEMBER FOR USE WITH LAWN MOWERS AND THELIKE" which issued on Mar. 6, 1990, and U.S. Pat. No. 5,023,998 toMasciarella, et al. entitled "FILAMENT HOLDER FOR A ROTATING FILAMENTVEGETATION CUTTER" which issued Jun. 18, 1991. This type of cutting headhas several drawbacks. First, the grooves and cut-outs include a numberof sharp corners which, in time, fray, weaken and break the stringdisposed therein. Second, a relatively thin string can only be used dueto the numerous twists and turns the string must pass through. As aresult, the thinner string breaks more often than thicker counter partsand requires repeated replacement. Third, disassembly of the cutter headis required to re-string the cutter head with subsequent bending andmanipulation of the string to conform with the grooves and cut-outsformed therein. A typical prior art cutting head requires 1-5 minutesfor disassembling and re-stringing. For a commercial gardener who spendseight to ten hours a day using a string trimmer, an hour or more may bewasted for re-stringing.

A second type of string trimmer cutting head design includes a spool ofstring enclosed within the cutting head, where the string is paid outthrough peripheral apertures formed in the circumference of the cuttinghead as required, as disclosed by U.S. Pat. No. 3,708,967 to Geist, etal. entitled "ROTARY CUTTING ASSEMBLY" which issued Jan. 9, 1973, U.S.Pat. No. 4,301,642 to Thurber entitled "SAFETY ROTOR FOR MOWER" whichissued Nov. 24, 1981, and U.S. Pat. No. 4,335,510 to Close, et al.entitled "STRING TRIMMER" which issued Jun. 22, 1982. The string isclamped within the cutting head to prevent unwanted extension therefrom.This cutting head design also has several drawbacks. A failure near anaperture may cause the end of the string to retract within the cuttinghead, thus requiring disassembly of the cutter head and re-threading ofthe string through the aperture. Although the cutting head may bedesigned to be readily removed, the common occurrence of a stringfailing adds up to a significant amount of time spent in disassemblingthe cutter head. Also, the operator of the string trimmer must carry theweight of an entire spool during the course of operation, which for acommercial gardener may be a substantial time and even an entire workday, resulting in a strenuous effort.

To overcome some of the shortcomings of the prior art, a third type ofstring trimmer cutter head design avoids the need for disassembly of thecutter head to accomplish re-stringing. For example, U.S. Pat. No.4,756,146 to Rouse entitled "STRING TRIMMER HEAD AND METHOD" whichissued Jul. 12, 1988 includes four arcuate channels, each extendingbetween two points on the periphery of the cutting head. The channelsare disposed in pairs so that each pair of channels frictionally engagesa string. The string is secured to the cutter head with one end of thestring being fed through each channel from a point external to thecutting head and with the ends being pulled through the channels so thatthe intermediate portion of the string comes into contact with a portionof the periphery of the cutting head disposed between the channels.Another example of the third type of cutting head design is disclosed inU.S. Pat. No. 5,398,416 to Mackey entitled "UNIVERSAL STRING TRIMMERREPLACEMENT HEAD" which issued Mar. 21, 1995 and which discloses acylindrical block formed with two non-parallel channels. A string issecured in the Mackey cutting head through a series of bends with thestring passing through the channels and engaging portions of the outersurface of the cutting head. Although these references avoid therequirement of disassembling a cutter head during the re-stringingprocess, the required number of bends and threading disclosed in thesereferences is also time-consuming. Due to the shortcomings of the priorart, there is a need for a string trimmer cutting head which may bequickly re-strung.

It is an object of this invention to provide a string trimmer cuttinghead which is easily and quickly re-strung.

It is also an object of this invention to provide a string trimmercutting head which can be easily and quickly strung with any gaugestring.

It is a further object of this invention to provide a string trimmercutting head with a centrifugally-driven clamping mechanism for holdingthe string within the cutting head which allows easy re-stringing of thecutting head.

It is yet another object of this invention to provide a lightweightstring trimmer cutting head which contains an amount of string neededonly for operation.

It is also another object of this invention to provide a string trimmercutting head which can be re-strung without disassembly or bending andtwisting of the string within the cutting head.

SUMMARY OF THE INVENTION

The above-stated objects are met by a new and improved string trimmercutting head which can be easily and quickly re-strung and is capable ofusing thin, as well as thick, gauge string.

The cutting head of the subject invention preferably includes asubstantially cylindrical body having a disc-shaped base plate formed todefine a central drive shaft aperture and a surrounding side wall. Aplurality of slots are formed in the side wall with a radially inwardextending pressing wall forming one edge of each of the slots. Aclamping member is pivotally mounted adjacent each aperture, oppositethe corresponding pressing wall so that its center of gravity isdisposed between the pivotal mounting and the corresponding pressingwall. The clamping members are adapted and formed to generate twodegrees of clamping force in cooperation with the corresponding pressingwalls. Springs are provided to generate one degree of clamping force,wherein the clamping force is sufficient to grippingly engage andmaintain strings within the cutting head while not in use. A second andgreater degree of clamping force is generated with the cutting headbeing in use, wherein the rotation of the cutting head createscentrifugal force that acts on the centers of gravity of the clampingmembers and enhances the gripping force thereof. The lesser degree ofclamping force can be overcome by a string forcibly introduced from alocation outside the cutting head, through the aperture and between theclamping member and the pressing wall. During operations the clampingmembers are rigidly locked into a clamping position from which thegreater degree of clamping force is generated. Since string may bethreaded through the apertures from a location outside the cutting head,no time-consuming disassembly of the cutting head is required tore-string the cutting head upon failure of a string. An operator of thesubject invention may carry a bundle of strings pre-cut to apre-determined length which can be easily threaded into the cutting headas needed.

In the preferred embodiment, the clamping members are formed with a camshape, and springs are provided to urge the cam-shaped clamping membersinto contact with the corresponding pressing wall. Each cam-shapedclamping member is formed and disposed with its center of gravity beinglocated between the pressing wall and the pivotal mounting. The springsgenerate the lesser degree of clamping force through the urging of theclamping members into contact with the corresponding pressing walls.During operation, the centrifugal force generated by the rotation of thecutting head acts on the centers of gravity of the clamping members,with resulting torques being generated in the clamping members about thepivotal mountings. Due to the location of the centers of gravity, thetorque generated in each clamping member forces the clamping member intogreater engagement with the corresponding pressing wall, therebyincreasing the clamping force generated by the clamping members and thepressing walls.

The apertures, pressing walls and clamping members are dimensioned toaccommodate thin, as well as thick, gauge string. Thus, the new andimproved cutting head of the subject invention is capable ofaccommodating relatively strong strings which reduce likelihood offailure. The accommodation of thicker strings, which reduces the numberof failures, and the ease of re-threading reduce an operator's lost timespent on re-stringing a string trimmer.

These and other features of the invention will be better understoodthrough a study of the following detailed description of the inventionand the accompanying drawings.

FIG. 1 is a perspective view of the new and improved cutting head of thesubject invention.

FIG. 2 is a plan view of the new and improved cutting head of thesubject invention.

FIG. 3 is a partial view of a string clamped by the new and improvedcutting head of the subject invention.

FIG. 4 is a partial view of a second embodiment of the new and improvedcutting head of the subject invention.

FIG. 5 is a partial view of a third embodiment of the new and improvedcutting head of the subject invention.

FIG. 6 is a partial view of a fourth embodiment of the new and improvedcutting head of the subject invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 the cutter head 10 is rotationally driven by amotor, not shown, through a drive shaft 12 which spins strings 14 at avelocity sufficient to cut vegetation. Unkept vegetation, such as a tuftof grass 16, may be cut to a desired height as shown by tuft 18 andblade of grass 20.

FIG. 2 more clearly shows the preferred embodiment of the cutting head10 which includes a disc-shaped base plate 22 formed with a central hub24. The base plate 22 is preferably formed with a circular shape.Alternatively, the base plate 22 can be formed with any shape thatallows the base plate 22 to rotate about its center in a balanced state.For example, the base plate 22 can be formed as a rectangle or a starwith equally-spaced points. Also, the cutting head 10 can be formed tovarious sizes by cutting the base plate 22 to the desired dimensions.

The hub 24 is frustoconically shaped and extends from the base plate 22in a single direction. A drive shaft aperture 26 is formed in the hub 24and dimensioned to allow a drive shaft to pass therethrough. The driveshaft may be secured to the hub 24 in the void formed by the hub 24above the plane of the base plate 22. A side wall 28 also extends fromthe base plate 22 in the same direction as the hub 24 to a height equalto or greater than the height of the hub 24. The side wall 28 isdiscontinuous with edges 30 and pressing walls 32 forming the ends ofeach length of the side wall 28.

The edges 30 and the pressing walls 32 both extend radially inwardtowards the hub 24 with the pressing walls 32 having a height equal tothat of the side wall 28. The edges 30 are formed with a height, asmeasured from the base plate 22, considerably less than the side wall28. The edges 30 and the pressing walls 32 are disposed to defineapertures 34 at opposing locations in the side wall 28. The apertures 34are each formed with a width capable of accommodating the string 14embodying any commonly used thickness. The side wall 28 may be formedwith any number of the apertures 34, so long the apertures 34 aredisposed at equal intervals along the length of the side wall 28. Theequal spacing of the apertures 34 is required to maintain the cuttinghead 10 balanced as the cutting head 10 rotates during use. As shown inFIG. 1, a cover 35 is provided which is formed to engage the side wall28 opposite the base plate 22 and prevent the introduction of dirt anddebris into the cutting head 10. The cover 35 is formed to allow a driveshaft to pass therethrough.

Referring to FIG. 2, cams 36 are pivotally mounted to post 38 whichextend from the edges 30. The cams 36 are mounted so that the topsurface of the cams is below the height of the pressing walls 32.Springs 40 are provided to bias the cams 36, with each of the springs 40having one end secured to one of the cams 36 and a second end secured toa bracket 42 formed in the side wall 28. The cams 36 are formed andmounted onto the posts 38 so that the center of gravity of each of thecams 36 is located at a point between the corresponding post 38 and thepressing wall 32, as shown in FIG. 3 with the center of gravityindicated by "C. G.". The springs 40 are secured to the cams 36 suchthat the cams 36 are urged in a clockwise direction into contact withthe pressing wall 32, as shown in FIG. 2. The springs 40 create thelesser degree of clamping force which is capable of maintaining thestring 14 between the cam 36 in the pressing wall 32. The spring forceof the spring 40 is selected to allow counter-clockwise rotation of thecams 36 with the insertion of the strings 14 into the cutting head 10from a location outside the side wall 28. The rotation of the cuttinghead 10 during use, in either a clockwise or counter-clockwisedirection, creates a centrifugal force which acts on the centers ofgravity of the cams 36 and creates torque about the posts 38 which causethe cams 36 to come into tighter engagement with the strings 14. Also,the centrifugal force radially extends the strings 14 and causes athinning effect with the cross-section of the strings 14 being slightlyreduced, thereby allowing the springs 40 and the torque acting on thecams 36 to urge the cams 36 into closer engagement with the pressingwalls 32 and create an even tighter engagement of the cams 36 and thepressing walls 32 about the strings 14.

In operation, the cutting head 10 is mounted onto a rotational driveshaft which passes through the cover 35 and the drive shaft aperture 26with the drive shaft being secured to the hub 24. One end of the string14 is guided through one of the apertures 34 formed in the side wall 28and forced between the cam 36 and the pressing wall 32. The passage ofthe string 14 between the cam 36 and the pressing wall 32 causes the camto rotate in a counter-clockwise direction. Once the string 14 is forcedinto the cutting head 10, no further force is required and the spring 40urges the cam 36 into clockwise rotation, resulting in the string 14being clamped between the cam 36 and the pressing wall 32. The string 14cannot be easily pulled out of the cutting head 10 since a force actingon the string 14 in a radially outward direction will also act on thecam 36 to cause it to rotate further clockwise and create a greaterclamping force.

With the cutting head 10 rotating in either direction, the clampingforce is enhanced due to centrifugal force created by the rotation ofthe cutting head 10 acting on the centers of gravity of the cams 36,which in turn, create torque about the post 38 and cause the cams 36 tocome into tighter engagement with the strings 14. If one of the strings14 fails during use, an operator of the cutting head 10 merely has toforce a new string into the cutting head 10. The subject invention doesnot require disassembly of the cutting head 10 to accomplishre-threading. The ends of the broken string are merely forced into thecutting head 10 without affecting the operation of the cutting head 10.Periodically, the cutting head 10 may be opened to dispose of thefragments of broken string.

In a second embodiment, torsional springs 44 may be employed about theposts 38 as shown in FIG. 4 in place of the coil springs 40. One end 46of the torsional spring 44 is secured to the edge 30 with the other end48 being secured to the cam 36 as shown in the cut-away section of FIG.4. The end 48 urges the cam 36 in a clockwise direction about the post38, with the operation of the second embodiment of the subject inventionbeing similar to that described above.

Alternatively, the cam 36 can be substituted for a different-shapedclamping member. As shown in FIG. 5, a cylindrical rod 50 may bepivotally mounted to the post 38 with its center of gravity locatedbetween the post 38 and the pressing wall 32. Any shaped clamping membermay be used with the subject invention so long as it is formed with itscenter of gravity disposed between the post 38 and the pressing wall 32and comprises an edge or face which can cooperate with the pressing wall32 to clamp the string 14.

In a fourth embodiment, the cutting head 10 may be formed with the cams36 capable of frictionally engaging the strings 14 without the use ofsprings 40. As shown in FIG. 6, the post 38 may extend through the edge30 or through the cover 35, not shown. A handle 52 is provided at theend of the post 38 opposite the cams 36. The cam 36 may be manuallyrotated about the post 38 by rotating the handle 52. The cam 36 forms anopening with the pressing wall having a width "D" which may be varied.The width "D" is measured between the closest points of the cam 36 andthe pressing wall 32 and represents the narrowest cross-section of theopening. The string 14 is formed with a diameter "d". To dispose thestring 14 within the cutting head 10 the cam 36 is rotated so that thewidth "D" of the opening is greater than the diameter "d" of the string14. Once within the cutting head 10 the cam 36 is rotated tofrictionally engage the string 14 and press the string 14 against thepressing wall 32, with the width "D" being less than the diameter "d" ofthe string 14. A plurality of spaced ribs 54 may be provided to enhancethe frictional engagement of the cam 36 with the string 14. In use, thecentrifugal force created by the rotation of the cutting head 10 acts onthe center of gravity of the cam 36, indicated by "C. G.", to enhancethe gripping engagement of the string 14. The rotation of the cuttinghead 10 causes the cam 36 to create a moment about the post 38 and clampstring 14 in tighter engagement. Also, the cams 36 can be substitutedfor different-shaped clamping member which is formed with a center ofgravity disposed between the post 38 and the pressing wall 32 during useand comprise an edge or face which can frictionally engage the string14.

As is readily apparent, numerous modifications and changes may readilyoccur to those skilled in the art, and hence it is not desired to limitthe invention to the exact construction and operation shown anddescribed, and accordingly all suitable modification equivalents may beresorted to falling within the scope of the invention as claimed.

What is claimed is:
 1. A combination of a cutting head and string forcutting and trimming vegetation, said combination being selectivelyrotationally driven through a drive shaft, said combinationcomprising:at least one generally straight length of string, each saidstring having two ends and an intermediate portion extendingtherebetween; a cutting head selectively rotationally driven by thedrive shaft, said head having a base plate and a side wall, said baseplate defining a periphery, said side wall extending from saidperiphery, said base plate and said side wall collectively defining avolume, said side wall being formed with at least one aperturecorresponding to each said string, wherein each said string is disposedto extend through a single said aperture with one said end of each saidstring being disposed in said volume defined by said base plate and saidside wall; and at least one clamping means for selectively fixing eachsaid string to said head.
 2. A combination as in claim 1, wherein eachsaid clamping means includes a pivotally mounted clamping member and apressing wall aligned to cooperatively clamp the string.
 3. Acombination as in claim 2, wherein each said clamping member beingformed with a center of gravity disposed between said pivotal mountingof said clamping member and cooperating pressing wall.
 4. A cutting headfor a string trimmer, said cutting head for accommodating at least onepiece of string, the string being fixedly supported in said cuttinghead, said cutting head comprising:base plate, said base plate defininga periphery; a side wall extending from the periphery of said baseplate, said side wall formed to define at least one aperture; at leastone wall means corresponding to each said aperture, each said wall meansfor at least partially supporting a single piece of the string, eachsaid wall means disposed adjacent a single said aperture; at least oneclamping member corresponding to each said aperture, each said clampingmember being pivotally mounted adjacent a single said aperture andopposite a single said wall means, each said clamping member having acontact surface for engagement with said opposing wall means; and atleast one biasing means corresponding to each said clamping member, eachsaid biasing means for both urging a single said clamping member intocontact with a corresponding opposing said wall means, and urging saidcontact surface of said clamping member in a direction towards saidaperture into engagement with an opposing said wall means, wherein eachpair of said clamping member and said opposing wall means fixedlysupports a single piece of the string in said cutting head.
 5. A cuttinghead as in claim 4, wherein at least one said biasing means includes acoil spring.
 6. A cutting head as in claim 4, wherein at least one saidbiasing means includes a torsional spring.
 7. A cutting head as in claim4, wherein the center of gravity of each said clamping member is locatedbetween said pivotal mounting of said clamping member and said opposingwall means.
 8. A cutting head as in claim 4, wherein at least one saidclamping member is formed as a cam.
 9. A cutting head as in claim 4,wherein at least one said clamping member is formed as a cylindricalrod.
 10. A cutting head as in claim 4, wherein at least one said wallmeans is formed continuously with said side wall and extends inwardlyfrom said side wall.
 11. A cutting head for string trimmers capable ofaccommodating any gauge string, said cutter head being rotationallydriven, said cutting head comprising:a base plate; a side wall extendingfrom the periphery of said base plate formed to define severalapertures, each having at least two opposed edges; at least one wallmeans for supporting the string disposed adjacent one of said opposededges of each said aperture; and a manually actuatable clamping meansfor clamping the string pivotally mounted adjacent each said apertureand opposite said at least one wall means, each said clamping meanshaving a manually actuated locked clamping position wherein the stringis clamped between said clamping means and said opposing at least onewall means, each said clamping means having a center of gravity locatedbetween said pivotal mounting and said opposing at least one wall meansso that the string is more tightly clamped by said clamping means andsaid at least one wall means with said clamping means being in saidlocked clamping position during the rotation of the cutter head.
 12. Acutting head as in claim 11, wherein each said clamping means includes aclamping member and the cutting head further comprising a handle meansfor urging said at least one clamping member into frictional engagementwith the string so as to clamp the string between said clamping memberand said opposing at least one wall means.
 13. A cutting head as inclaim 12, wherein said clamping member is formed as a cam.
 14. A cuttinghead as in claim 12, wherein said clamping member is formed with aplurality of spaced ribs.
 15. A cutting head for a string trimmer, saidcutting head for accommodating at least one piece of string, the stringbeing fixedly supported in said cutting head, said cutting headcomprising:a base plate, said base plate defining a periphery; a sidewall extending from the periphery of said base plate, said side wallformed to define at least one aperture; at least one wall meanscorresponding to each said aperture, each said wall means for at leastpartially supporting a single piece of the string, each said wall meansdisposed adjacent a single said aperture; at least one clamping membercorresponding to each said aperture, each said clamping member beingpivotally mounted adjacent a single said aperture and opposite a singlesaid wall means, each said clamping member having a contact surface forengagement with the string; and at least one spring corresponding toeach said clamping member, each said spring urging said contact surfaceof said clamping member in a direction towards the opposing wall meansand said aperture, which is adjacent said clamping member, and whereineach pair of said clamping member and said opposing wall means fixedlysupports a single piece of the string in said cutting head.
 16. Acutting head as in claim 15, wherein the center of gravity of each saidclamping member is located between said pivotal mounting of saidclamping member and said opposing wall means.
 17. A cutting head as inclaim 15, wherein at least one said clamping member is formed as a cam.18. A cutting head as in claim 15, wherein at least one said clampingmember is formed with a plurality of spaced ribs.
 19. A cutting head asin claim 15, wherein at least one said wall means is formed continuouslywith said side wall and extends inwardly from said side wall.